In the field of semiconductor devices, a MOSFET is relatively essential and popular. For a MOSFET, it is desirable to miniaturize the device without sacrificing its operational properties. Changing the channel width and/or length of the MOSFET is one of the approaches to reduce the device size. Please refer to FIG. 1 which schematically shows a top-view shape of a conventional mask applied to a MOSFET having a narrow and short channel. In FIG. 1, the rectangular mask regions 11 and 12 define respective active regions, and the mask region 13 defines a region for forming therein a channel. Another region 14 schematically shows the distribution of a poly gate conductive layer. On the other hand, the remaining non-mask region is for forming field oxides for isolation. In the figure, the label L indicates the length of the channel, and the label W represents the width of the channel.
Now refer to FIG. 2 which is a cross-sectional view taken along the line X-X' of FIG. 1. For a mask shape and channel dimensions as shown in FIG. 1, the field oxide layers 21 and 22 formed beside the region 13 will have a cross-sectional shape as shown in FIG. 2. That is, portions 211 and 221 of the field oxide layers 21 and 22 beside the channel region 13 will become relatively thin due to a three dimensional oxidation thinning effect. The threshold voltage of the portions 211 and 221 of the field oxide layers 21 and 22 beside the channel region 13 will be accordingly reduced, and thus this isolation region is subject to the formation of an extensive channel so that the effective channel width is greater than the predetermined one. Therefore, the operational properties of the device will be somewhat different from the expected ones. It is particular obvious when the channel of the MOSFET is as narrow as less than 0.5 microns, and as short as less than 0.5 microns.
FIG. 3 is a saturate drain current per unit width vs. channel width plot of a MOSFET having a narrow and short channel and a layout as described above on the conditions of channel lengths of 25, 2, 1, 0.8, 0.6 and 0.45 microns, respectively, wherein four kinds of channel widths, i.e. 25, 3, 1 and 0.4 microns, are taken as measured points for each channel length. It is apparent from FIG. 3 that the drain current per .mu.m width (IDsat/W in amp/.mu.m) increases with the decrease of the channel width (W in .mu.m), and the shorter the channel length, the more significant the effect of W on IDsat/W. In other words, owing to the 3-D oxidation thinning effect of the field isolation region beside the very short channel, which results from the rectangular shape of the mask on the active regions, an extensive channel is likely to be formed in the isolation region so that the effective channel width will be greater than the predetermined one, and the drain current will be varied beyond expectation. It will be a problem for the miniaturization of the device.